The study focuses on the nature of the reactions given by human babies during the second year to a variety of unfamiliar situations, on their consistency within this period, and on the persistence of individual patterns at age 3-1/2 years. Data are derived from (a) video records of the behaviors of 40 male and female babies which document their initial reactions and ultimate accommodation to a set of experimentally devised semi-naturalistic situations, including encounters with novel objects, strange adults, and unfamiliar age-peers; and (b) from ratings of reactions to strangers in the home made repeatedly during the second year and again at age 3-1/2, and of reactions observed in a nursery school setting at 3-1/2 years. The aims are (1) to document the form and frequency with which wariness is expressed in a variety of typically occurring new situations; (2) to determine the degree to which individual infants show consistent differences in reactions given to a variety of new situations as well as over time; and (3) through detailed analyses of moment-to- moment behaviors shown in single episodes to examine the dynamic processes through which conflicting tendencies to explore and to withdraw find their resolution.